1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, and more specifically, an image forming apparatus wherein a multicolored image of a multicolored original is formed on sheets in one cycle of image forming operation based on the electrophotographic processing.
2. Description of Related Art
Multicolored originals to be subjected to a copying operation are generally those which have black letters marked with a color marker and corrections in color ink. A variety of methods and apparatuses for copying a bicolored image of such an original on a sheet in one cycle of the image forming operation (including one cycle of a charging process, an exposing process and a transferring process) have been discussed and developed. For example, Japanese patent Laid Open Publication No. 55-38561 discloses a high-speed multicolor printing method. According to the publication, after an electrostatic latent image is formed having two values, a plurality of developing devices are impressed with respective developing bias voltages different from each other so that each portion of the latent image having a different value can be developed.
In a method for developing an electrostatic latent image having two values by applying different developing bias voltages, if the density is set high so that the marked portions are developed densely, the marked portions and lettered portions will be mixed with each other in the border portions. Because of this, the contrast of the letters and the mark will be weaken, and there will be a difficulty in reading the lettered portions packed with primary information. On the other hand, if the density is set low so that the marked portions are developed thinly corrected portions will be developed thinly, too, so that there will be a difficulty in reading the correction.